This project will evaluate the effects of illicit drugs on central nervous system dopamine function. Positron emission tomography with 6-(18F)-L-DOPA(FD) will determine forward and reverse transport across the blood-brain barrier, decarboxylation to 18F-dopamine (FDA), and release and metabolism of FDA. One group of 81 individuals has been exposed to the dopaminergic neurotoxin MPTP. second group of test subjects are chronic high-dose cocaine abusers.